Disperson-shifted optical fiber employing dual shape core profile

ABSTRACT

A dispersion-shifted optical fiber is obtained that has a refractive index distribution shape comprising a center core portion of high refractive index, a step core portion of lower refractive index than the center core portion, provided about the outer circumference thereof, and clad of lower refractive index than the step core portion, provided about the outer circumference of the step core, in which, by suitably setting structural parameters, both Aeff expansion and dispersion slope reduction are satisfactorily effected, simultaneously, under conditions that the optical fiber is substantially single-mode, and bending loss is held down to 100 dB/m or lower.

TECHNICAL FIELD

This invention relates to dispersion-shifted optical fiber having largeeffective core area and low dispersion slope.

The present invention is based on patent applications filed in Japan(Japanese Patent Application No. H11-212949/1999, Japanese PatentApplication No. H11-230137/1999, Japanese Patent Application No.2000-64008, Japanese Patent Application No. 2000-224491, and JapanesePatent Application No. 2000-224492), and the particulars described inthose Japanese patent applications are incorporated as part of thisspecification.

BACKGROUND ART

In a long-haul system such as an optical amplifier repeater transmissionsystem employing optical fiber amplifiers, it is important that thenonlinear optical effects be reduced. A parameter called the nonlinearoptical coefficient is a parameter that serves as an index to the degreeof nonlinear optical effect. The nonlinear optical coefficient isexpressed by n2/Aeff, where n2 is the nonlinear refractive index andAeff is the effective core area. The value of n2 becomes roughlyconstant depending on the material, and expanding the Aeff is aneffective technique for reducing nonlinear optical effects.

In wavelength division multiplexed transmission systems which can handlehigh-volume transmission, on the other hand, there is a need to suppressthe chromatic dispersion value and reduce dispersion slope. It is wellknown that, in a wavelength division multiplexed transmission system,when a zero-dispersion wavelength exists in the transmission bandwidth,transmission quality declines due to a nonlinear optical effect calledfour-wave mixing. However, because large chromatic dispersion values areaccompanied by signal waveform deterioration, it is necessary tosuppress that value to a certain size. In order to satisfy theseconflicting demands, optical fiber called non-zero dispersion-shiftedoptical fiber in which the chromatic dispersion value in the wavelengthband used is controlled to within a narrow range has been developed.

In a wavelength division multiplexed transmission system, furthermore,reducing the dispersion slope is also important. By dispersion slope,which indicates the wavelength dependency of the chromatic dispersionvalue, is meant the slope of the curve obtained by plotting wavelength(nm) on the horizontal axis and chromatic dispersion value (ps/km·nm) onthe vertical axis. In a wavelength division multiplexed transmissionsystem, if the dispersion slope of the transmission line (optical fiber)is large, the difference in chromatic dispersion value betweenwavelengths will be great. For that reason, by taking a very largedispersion value, depending on the wavelength, difficulties areencountered such as the transmission quality being greatly differentbetween different channels. Accordingly, there is a need to make thedispersion slope smaller.

The specific values for the characteristics demanded in the Aeff anddispersion discussed above will be different according to the systememployed. In a system in which transmissions are made over very longhaul, such as submarine systems, a reduction in nonlinear optical effectresulting from Aeff expansion is sought. In a system extending for fromseveral tens to several hundreds of km, on the other hand, there issometimes a need to suppress the dispersion value in a wide wavelengthband by dispersion slope reduction. In terms of the minimum conditionsdemanded for the transmission line in a light communication system,furthermore, the optical fiber should be substantially single-mode, andthe bending loss should be held down to 100 dB/m or lower.

That being so, proposals have recently been made on ways to effect somedegree of Aeff expansion and dispersion slope decrease using variousrefractive index distribution shapes (refractive index profiles), as inJapanese Patent Application Laid-Open No. H10-62640/1998, JapanesePatent Application Laid-Open No. H10-293225/1998, Japanese PatentApplication Laid-Open No. H8-220362/1996, and Japanese PatentApplication Laid-Open No. H10-246830/1998, for example.

In FIGS. 10A to 10C are diagrammed examples of shapes of refractiveindex distribution of such dispersion-shifted optical fiber.

In FIG. 10A is represented one example of a dual shape core (step) typeof refractive index distribution, in a core 14 is formed, with thesymbol 11 designating the center core portion and a step core portion 12provided about the outer circumference thereof having a lower refractiveindex than the center core portion 11. Furthermore, about the outercircumference of that core 14, clad 17 is provided having a lowerrefractive index than the step core portion 12.

In Japanese Patent Application Laid-Open No. H8-220362/1996, the presentapplicant disclosed the use of the smaller diameter solution, for thepurpose of expansion of Aeff, in dispersion-shifted optical fiber havinga dual shape core type refractive index distribution.

It has been known for some time that, when the core diameter of adispersion-shifted optical fiber is expanded while maintaining thesimilarity of refractive index distribution shape, two or more solutionsexist wherewith the chromatic dispersion value becomes the desiredvalue. At such time, of the solutions within a range wherein the bendingloss and cutoff wavelength characteristics are comparatively practical,the solution wherewith the core diameter is relatively thin is calledthe smaller diameter solution, and the solution wherewith the corediameter is relatively large is called the larger diameter solution.

In FIG. 10B is represented an example of a segmented core type ofrefractive index distribution shape, wherein a core 24 is configuredwith an intermediate portion 22 of low refractive index provided aboutthe outer circumference of the center core portion 21 of high refractiveindex, and a ring core portion 23 having a higher refractive index thanthe intermediate portion 22 but a lower refractive index than the centercore portion 21 provided about the outer circumference of thatintermediate part 22. Also, about the outer circumference of that ringcore portion 23 is provided a first clad 25 having a lower refractiveindex than the intermediate portion 22, and about the outercircumference of that first clad 25 is provided a second clad 26 havinga higher refractive index than the first clad 25 but a lower refractiveindex than the intermediate portion 22, thus configuring clad 27.

In Japanese Patent Application Laid-Open No. H11-119045/1999(published), furthermore, the present applicant disclosed adispersion-shifted optical fiber well suited to optical communicationsystems wherein the reduction of the dispersion slope is more rigorouslydemanded than the expansion of the Aeff, by using the larger diametersolution in a segmented core type of refractive index distributionshape.

In FIG. 10C is represented an example of an O ring type refractive indexdistribution shape, wherein a core 34 is configured with a two-layerstructure, with a peripheral core portion 32 of high refractive indexprovided about the outer circumference of a center core portion 31 oflow refractive index at the center. About the outer circumference ofthat core 34 is provided clad 37 of lower refractive index than theperipheral core portion 32, thereby configuring a three-layer structureconvex type refractive index distribution shape inclusive of the clad37.

In the dispersion-shifted optical fibers conventionally proposed,however, under such conditions as that they are substantiallysingle-mode and that the bending loss is held down to 100 dB/m or lower,it is very difficult to sufficiently realize both Aeff expansion anddispersion slope reduction simultaneously.

Looking at the dual shape core type of optical fiber wherein the smallerdiameter solution is used disclosed in Japanese Patent ApplicationLaid-Open No. H8-220362/1996, for example, the dispersion slope is inthe neighborhood of 0.10 ps/km/nm² at minimum, wherefore this opticalfiber is sometimes inadequate for use in systems where dispersion slopereduction is rigorously demanded.

With the segmented core type optical fiber wherewith the larger diametersolution is used, disclosed in Japanese Patent Application Laid-Open No.H11-119045/1999, characteristics close to those demanded in somewhatmore recent wavelength division multiplexed transmission systems areobtained. However, because the refractive index distribution shapecomprises a five-layer structure wherein the refractive index increasesand declines, the characteristics vary subtly depending on the position,width, and shape, etc., of each layer. Accordingly, during manufacture,a high level of controllability is demanded for such structuralparameters as the radius and relative refractive-index difference ofeach layer. As a consequence, there is a limit to the degree to whichproduct yield can be improved.

With the increase in the number of channels (i.e. number of multiplexedwavelengths), moreover, dispersion-shifted optical fiber has come to bedemanded which can be employed all across a wide transmission wavelengthband of 1490 to 1625 nm in which the so-called L band (1570 to 1610 nm)has been added.

Conventional dispersion-shifted optical fiber of expanded Aeff isdesigned with transmission in the 1550 nm band in view, wherefore suchoptical fiber having adequate characteristics in the L band has not beenprovided. In many cases, bending loss became large particularly in the Lband.

An object of the present invention, which was devised in view of thecircumstances described above, is to provide dispersion-shifted opticalfiber wherewith Aeff expansion and dispersion slope reduction can bothbe satisfactorily realized, simultaneously, under conditions such that asingle-mode is substantially realized and the bending loss is held to100 dB/m or less.

Another object is to provide dispersion-shifted optical fiber wherewithstabilized characteristics are exhibited with a structure made as simpleas possible, that can nevertheless be efficiently manufactured.

Another object is to provide dispersion-shifted optical fiber wherewithAeff expansion and dispersion slope reduction can both be satisfactorilyrealized, simultaneously, under conditions such that a single-mode issubstantially realized and the bending loss is held to 100 dB/m or less,even in a broad wavelength band to which the L band has been added,covering from 1490 to 1625 nm.

Another object is to provide dispersion-shifted optical fiber thatexhibits low bending loss, particularly in the L band.

DISCLOSURE OF THE INVENTION

In order to realize the objects stated above, a first dispersion-shiftedoptical fiber of the present invention is dispersion-shifted opticalfiber having a refractive index distribution shape comprising a centercore portion of high refractive index, a step core portion of lowerrefractive index than the center core portion, provided about the outercircumference thereof, and clad of lower refractive index than the stepcore portion, provided about the outer circumference of the step coreportion, in which, the dispersion-shifted optical fiber has, in a usedwavelength band selected from 1490 to 1625 nm, Aeff of 45 to 90 μm²,dispersion slope of from 0.05 to 0.14 ps/km/nm², bending loss of 100dB/m or less, and chromatic dispersion value of either from −0.5 to −8.0ps/km/nm or from +0.05 to +10.0 ps/km/nm, and has a cutoff wavelengthsuch that substantially single-mode propagation is realized.

A second dispersion-shifted optical fiber is characterized in that, inthe first dispersion-shifted optical fiber, larger diameter solution isadopted for core diameter, and the dispersion-shifted optical fiber has,in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to70 μm², dispersion slope of from 0.05 to 0.08 ps/km/nm², bending loss of100 dB/m or less, and chromatic dispersion value of from −0.5 to −8.0ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized.

A third dispersion-shifted optical fiber is characterized in that, inthe second dispersion-shifted optical fiber, when radius of the centercore portion is represented as r1, radius of the step core portion asr2, relative refractive-index difference of the center core portion whenrefractive index of outermost clad is taken as reference as Δ1, andrelative refractive-index difference of the step core portion as Δ2,r2/r1 is from 4 to 12, Δ2/Δ1 is from 0.05 to 0.15, and Δ1 is from 0.55to 0.85%.

A fourth dispersion-shifted optical fiber is characterized in that, inthe second dispersion-shifted optical fiber, the clad comprises firstclad provided about outer circumference of said step core portion andsecond clad having a higher refractive index than the first clad,provided about outer circumference of the first clad.

A fifth dispersion-shifted optical fiber is characterized in that, inthe fourth dispersion-shifted optical fiber, when radius of the centercore portion is represented as r1, radius of the step core portion asr2, radius of the first clad as r3, relative refractive-index differenceof the center core portion when refractive index of the outermost cladis taken as reference as Δ1, relative refractive-index difference of thestep core portion as Δ2, and relative refractive-index difference of thefirst clad as A3, r2/r1 is from 4 to 12, Δ2/Δ1 is from 0.05 to 0.15, Δ1is from 0.55 to 0.85%, Δ3 is from −0.3 to 0%, and (r3−r2)/r1 is from 0.2to 4.0.

A sixth dispersion-shifted optical fiber is characterized in that, inthe first dispersion-shifted optical fiber, larger diameter solution isadopted for core diameter, and the dispersion-shifted optical fiber has,in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to70 μm², dispersion slope of from 0.05 to 0.075 ps/km/nm²; bending lossof 100 dB/m or less, and chromatic dispersion value of from +0.05 to+10.0 ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized.

A seventh dispersion-shifted optical fiber is characterized in that, inthe sixth dispersion-shifted optical fiber, when radius of the centercore portion is represented as r1, radius of the step core portion asr2, relative refractive-index difference of the center core portion whenrefractive index of the outermost clad is taken as reference as Δ1, andrelative refractive-index difference of the step core portion as Δ2,r2/r1 is from 4 to 12, Δ1 is from 0.55 to 0.75%, and Δ2/Δ1 is from 0.05to 0.15.

An eighth dispersion-shifted optical fiber is characterized in that, inthe sixth dispersion-shifted optical fiber, the clad comprises firstclad provided about outer circumference of the step core portion andsecond clad provided about outer circumference thereof.

A ninth dispersion-shifted optical fiber is characterized in that, inthe eighth dispersion-shifted optical fiber, when radius of the centercore portion is represented as r1, radius of the step core portion asr2, radius of the first clad as r3, relative refractive-index differenceof the center core portion when refractive index of the second clad istaken as reference as Δ1, relative refractive-index difference of thestep core portion as Δ2, and relative refractive-index difference of thefirst clad as Δ3, r2/r1 is from 4 to 12, Δ1 is from 0.55 to 0.75%, Δ2/Δ1is from 0.05 to 0.15, Δ3 is from −0.1 to 0%, and (r3−r2)/r1 is from 0.2to 4.0.

A tenth dispersion-shifted optical fiber is characterized in that, inthe first dispersion-shifted optical fiber, smaller diameter solution isadopted for the core diameter, and the dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 65to 95 μm², dispersion slope of from 0.08 to 0.14 ps/km/nm², bending lossof 100 dB/m or less, and the absolute values of the chromatic dispersionvalue of from 0.5 to 8.0 ps/km/nm, and has a cutoff wavelength such thatsubstantially single-mode propagation is realized.

An 11th dispersion-shifted optical fiber is characterized in that, inthe tenth dispersion-shifted optical fiber, when radius of the centercore portion is represented as r1, radius of the step core portion asr2, relative refractive-index difference of the center core portion whenrefractive index of the clad is taken as reference as Δ1, relativerefractive-index difference of the step core portion as Δ2, r2/r1 as x,and Δ2/Δ1 as y, 5≦x≦10, 0.08≦y≦0.22, and 0.6%≦Δ1≦1.2%.

A 12th dispersion-shifted optical fiber is the tenth dispersion-shiftedoptical fiber having a zero dispersion wavelength on the side of longerwavelengths than the wavelength band used.

A 13th dispersion-shifted optical fiber is characterized in that, in the12th dispersion-shifted optical fiber, when radius of the center coreportion is represented as r1, radius of the step core portion as r2,relative refractive-index difference of the center core portion whenrefractive index of the clad is taken as reference as Δ1, relativerefractive-index difference of the step core portion as Δ2, r2/r1 as x,and Δ2/Δ1 as y, 6≦x≦7, 0.1≦y≦0.18, y≧(−0.02x+0.24), 0.6%≦Δ1≦1.2%, Aeffis from 65 to 75 μm², and dispersion slope is 0.125 ps/km/nm² or less.

A 14th dispersion-shifted optical fiber is characterized in that, in the12th dispersion-shifted optical fiber, 7≦x≦8, 0.1≦y≦0.16, y≧(−0.016x+0.21), 0.6%≦Δ1≦1.2%, Aeff is from 70 to 80 μm², and dispersion slope is0.130 ps/km/nm² or less.

A 15th dispersion-shifted optical fiber is characterized in that, in the12th dispersion-shifted optical fiber, 7≦x≦8.5, 0.1≦y≦0.16,(−0.02x+0.26)≦y≦(−0.02x+0.32), 0.6%≦Δ1≦1.2%, Aeff is from 75 to 85 μm²,and dispersion slope is 0.135 ps/km/nm² or less.

A 16th dispersion-shifted optical fiber is the tenth dispersion-shiftedoptical fiber having a zero dispersion wavelength on the side of shorterwavelength than the wavelength band used.

A 17th dispersion-shifted optical fiber is characterized in that, in the16th dispersion-shifted optical fiber, when radius of the center coreportion is represented as r1, radius of the step core portion as r2,relative refractive-index difference of the center core portion whenrefractive index of the clad is taken as reference as Δ1, relativerefractive-index difference of the step core portion as Δ2, r2/r1 as x,and Δ2/Δ1 as y, 5≦x≦8, 0.12≦y≦0.22, (−0.02x+0.24)≦y≦(−0.02x+0.34),0.6%≦Δ1≦1.2%, Aeff is from 65 to 75 m², and dispersion slope is 0.110ps/km/nm² or less.

An 18th dispersion-shifted optical fiber is characterized in that, inthe 16th dispersion-shifted optical fiber, 5.5≦x≦8, 0.12≦y≦0.20,(−0.02x+0.25)≦y≦(−0.02x+0.33), 0.6%≦≦Δ1≦1.2%, Aeff is from 70 to 80 μm²,and dispersion slope is 0.115 ps/km/nm² or less.

A 19th dispersion-shifted optical fiber is characterized in that, in the16th dispersion-shifted optical fiber, 6≦x≦8, 0.12≦y≦0.20,(−0.02x+0.26)≦y≦(−0.02x+0.35), 0.6%≦Δ1≦1.2%, Aeff is from 75 to 85 μm²,and dispersion slope is 0.125 ps/km/nm² or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of a first example of a refractive indexdistribution shape for a dispersion-shifted optical fiber of the presentinvention;

FIG. 1B is a diagram of a second example of a refractive indexdistribution shape for a dispersion-shifted optical fiber of the presentinvention;

FIG. 2 is a graph representing an example analysis when the firstexample of the refractive index dispersion shape diagrammed in FIG. 1Ais used in a first embodiment;

FIG. 3 is a graph representing an example of the dependency of chromaticdispersion values on wavelength for a dispersion-shifted optical fiberrelating to the first embodiment;

FIG. 4 is a graph representing the variation in the bending lossaccording to combinations of values of Δ3 and (r3−r2)/r1 when the secondexample of the refractive index distribution shape diagrammed in FIG. 1Bis used in the first embodiment;

FIG. 5 is a graph representing an example analysis when the refractiveindex distribution shape diagrammed in FIG. 1A is used in a secondembodiment;

FIGS. 6A and 6B are graphs plotting the variation in bending loss andAeff, respectively, according to combinations of values of Δ3 and(r3−r2)/r1 when the second example of refractive index distributionshape diagrammed in FIG. 1B is used in the second embodiment;

FIG. 7 is a graph of analysis results representing the tracks of thesmaller diameter solutions when the values of Δ2/Δ1 and Δ1 are varied atvalues for r2/r1 of 5.0, 7.0, and 9.0, respectively, in a thirdembodiment;

FIG. 8 is a graph representing distributions of characteristic valuesassociated with changes in Δ2/Δ1 and Δ1 when r2/r1 is 7.0 in the thirdembodiment;

FIG. 9 is a graph representing distributions of characteristic valuesassociated with changes in Δ2/Δ1 and Δ1 when r2/r1 is 9.0 in the thirdembodiment;

FIG. 10A is a diagram of an example of a dispersion-shifted opticalfiber's refractive index distribution shape of the background art;

FIG. 10B is a diagram of an example of a dispersion-shifted opticalfiber's refractive index distribution shape of the background art;

FIG. 10C is a diagram of an example of a dispersion-shifted opticalfiber's refractive index distribution shape of the background art.

BEST MODE FOR CARRYING OUT THE INVENTION

The dispersion-shifted optical fiber of the present invention has arefractive index distribution shape comprising a center core portion ofhigh refractive index, a step core portion of lower refractive indexthan the center core portion, provided about the outer circumference ofthe center core portion, and clad of lower refractive index than thestep core portion, provided about the outer circumference of the stepcore portion.

By adjusting the structural parameters, moreover, a dispersion-shiftedoptical fiber is obtained wherein, in an used wavelength band selectedfrom a range of 1490 to 1625 nm, the Aeff is 45 to 90 μm², thedispersion slope from 0.05 to 0.14 ps/km/nm², the bending loss 100 dB/mor less, and the chromatic dispersion value either from −0.5 to −8.0ps/km/nm or from 0.05 to 10.0 ps/km/nm, which has a cutoff wavelengthsuch that substantially single-mode propagation is realized.

The present invention is now described in detail in terms of first,second, and third embodiments.

First Embodiment

FIG. 1A represents a first example of a refractive index distributionshape for a dispersion-shifted optical fiber in this first embodiment.

This refractive index distribution shape is configured by a core 4wherein a step core portion 2 is provided about the outer circumferenceof a center core portion 1, and clad 7 of a single layer structurehaving a uniform refractive index, provided about the outercircumference of the core 4.

The center core portion 1 has the highest refractive index, the stepcore portion 2 has a refractive index lower than that of the center coreportion 1, and the clad 7 has a refractive index lower than that of thestep core portion 2.

The symbols r1 and r2 in figure, respectively, indicate the radiuses ofthe center core portion 1 and the step core portion 2, while Δ1 and Δ2,respectively, indicate the relative refractive-index difference of thecenter core portion 1 and the relative refractive-index difference ofthe step core portion 2 when the refractive index of the clad 7 is usedas a reference.

In this example, the center core portion 1 and the step core portion 2are configured by germanium-doped quartz glass to which has been addedgermanium which exhibits an effect to raise the refractive index, whilethe clad 7 is configured by pure quartz glass.

In the refractive index distribution shape of the dispersion-shiftedoptical fiber, furthermore, the boundary between each layer (i.e. thecenter core portion 1, step core portion 2, and clad 7) need not bedefinite, as diagrammed in FIG. 1A, but may instead be in a roundedcondition exhibiting so-called sagging, and need not be particularlylimited so long as the characteristics of the dispersion-shifted opticalfiber of this embodiment are effectively exhibited.

In the dispersion-shifted optical fiber of this embodiment, a wavelengthrange extending from 1490 to 1625 nm, and generally from 1490 to 1610nm, is made the main wavelength band for use, and a wavelength bandhaving a suitable wavelength width is selected from these ranges whendetermining the embodiment specifications. These wavelength bands arelargely classified into three wavelength bands according to theamplification wavelength band based on the optical fiber amplifiers usedin the light communication system. More specifically, it is common todesignate the wavelength band extending from 1490 to 1530 nm as the Sband, the wavelength band extending from 1530 to 1565 nm as the C band,and the wavelength band extending from 1565 to 1625, but generally from1490 to 1610 nm, as the L band. The systems currently in use primarilyemploy the C band, but systems are being developed which presume the useof the L band in addition to the C band in order to respond to demandsfor bands exhibiting increased transmission volume.

The Aeff is found by the following formula.${Aeff} = \frac{2\pi \left\{ {\int_{0}^{\infty}{a{{E(a)}}^{2}\quad {a}}} \right\}^{2}}{\int_{0}^{\infty}{a{{E(a)}}^{4}\quad {a}}}$

where a is the core radius and E(a) is the electric field strength atthe radius a.

In this embodiment, when the Aeff in the wavelength band used is lessthan 45 μm² the nonlinear optical effect is insufficiently suppressed.Dispersion-shifted optical fiber wherein the Aeff exceeds 70 μm² is verydifficult to manufacture.

The smaller the value of the dispersion slope in the wavelength bandused the better, as noted in the foregoing. In this embodiment, it ispossible to realize very small values for the dispersion slope in thewavelength band used, namely from 0.05 to 0.08 ps/km/nm². When 0.08ps/km/nm² is exceeded, the wavelength dependency of the chromaticdispersion value becomes great, which with this embodiment sometimespresents difficulties when applied in wavelength division multiplexedsystems. At values less than 0.05 ps/km/nm², manufacture is verydifficult.

By bending loss is meant a value occurring in the wavelength band usedunder the condition wherein the bend diameter (2R) is 20 mm.

The smaller the bending loss the better. In this embodiment, the bendingloss is made 100 dB/m or less, and preferably 40 dB/m or less. When 100dB/m is exceeded, transmission loss readily worsens due to slight bendsimparted to the dispersion-shifted optical fiber, and unnecessary lossesoccur during laying operations or other handling, which is problematic.

In this embodiment, the chromatic dispersion value is made to be withina range of −0.5 to −8.0 ps/km/nm. When this value is larger than −0.5ps/km/nm, the chromatic dispersion value approaches zero, and four-wavemixing, which is one nonlinear optical effect, readily occurs, which isproblematic. When the value is smaller than −8.0 ps/km/nm,dispersion-induced waveform distortion occurs, and transmissioncharacteristic deterioration becomes great, which is problematic.However, the range of dispersion values allowable in practice may varydepending on the relay distance and other system design factors.

Furthermore, because the dispersion-shifted optical fiber in thisembodiment is a single-mode optical fiber, it is necessary to have acutoff wavelength that substantially guarantees single-mode propagationin the wavelength band used.

Ordinary cutoff wavelengths are defined by values based on the CCITT's2m method (hereinafter called the 2m method). However, in actuallong-length use conditions, single-mode propagation is possible evenwhen this value is on the longer wavelength side from the lower limitingvalue in the wavelength band used.

That being so, in the dispersion-shifted optical fiber of thisembodiment, the cutoff wavelength defined by the 2m method is set sothat single-mode propagation is possible according to the length of thedispersion-shifted optical fiber used and the wavelength band used. Morespecifically, if the cutoff wavelength in the 2m method is 1800 nm orless, under length conditions of about 5000 m or more, it is possible toeffect single-mode propagation in the wavelength band used as describedin the foregoing.

A configuration for satisfying such characteristics as these isdescribed below, together with the research history thereof.

First, in this embodiment, the larger diameter solution is used as thecore diameter, as described earlier. More specifically, in settingstructural parameters that satisfy the numerical ranges for r2/r1,Δ2/Δ1, and Δ1, described below, using simulations, the settings are madeso that the core diameter becomes the larger diameter solution, anddesign conditions are established which satisfy such characteristicvalues as Aeff, dispersion slope, and so on, in the wavelength banddesired for use, as described in the foregoing. Furthermore, in terms ofthe actual method of manufacturing the dispersion-shifted optical fiberof this embodiment, a conventional method such as CVD, VAD, and so on,can be used.

FIG. 2 is a graph representing an example of an analysis made when thisfirst example refractive index distribution shape is used.

The values 5, 7, and 10 corresponding respectively to the symbols ⋄, Δ,and + represented in the graph are values for r2/r1 (step magnificationfactor) that is the ratio between the radiuses of the center coreportion 1 and the step core portion 2 diagrammed in FIG. 1A. Aeff isplotted on the horizontal axis and dispersion slope on the verticalaxis, both of which are values for the wavelength 1550 nm.

From this graph it is seen that Aeff tends to expand as the value ofr2/r1 increases, while the dispersion slope tends to become smaller. Inorder to satisfy the numerical value ranges for the chromatic dispersionvalue and bending loss noted earlier, it is preferable that r2/r1 be setat 4 times or greater. At less than 4 times, it becomes very difficultto realize characteristics that are better than those of conventionaldispersion-shifted optical fiber. When 12 times is exceeded,productivity declines, which is problematic.

It is also desirable that Δ2/Δ1 be within a range of 0.05 to 0.15. Atless than 0.05, bending loss becomes large, which is problematic. When0.15 is exceeded, the cutoff wavelength becomes long, whereupon, in somecases, single-mode transmission cannot be sustained.

Δ1 is made to be within a range of 0.55 to 0.85%. When this value isless than 0.55%, it becomes very difficult to set the wavelength bandused within the range of −0.5 to −8.0 ps/km/nm. When Δ1 is made large,it becomes possible to make the dispersion value small, but when 0.85%is exceeded, it becomes impossible to make Aeff sufficiently large,which is problematic.

Design is effected, making selections for the combinations of thenumerical values from these numerical ranges for r2/r1, Δ2/Δ1, and Δ1,so as to satisfy the characteristics of the dispersion-shifted opticalfiber of this embodiment.

In the dispersion-shifted optical fiber of this embodiment, furthermore,r2, that is, the core radius, is not limited particularly. Ordinarilythis value will be in a range of 10 to 25 μm. The outer diameter of theclad 7 is ordinarily made approximately 125 μm.

In Table 1, specific design examples of dispersion-shifted opticalfibers that satisfy such conditions as these are indicated. In thistable, kcf represents the fiber cutoff wavelength based on the 2mmethod, λop the wavelength at which the characteristics are measured,and MFD the mode field diameter.

In each of these examples, characteristics are obtained which satisfythe preferred numerical ranges for Aeff, dispersion slope, chromaticdispersion value, bending loss, and cutoff wavelength, such as aresuitable for a wavelength division multiplexed transmission system.

The graph (a) given in FIG. 3 represents an example of the wavelengthdependency of the chromatic dispersion value in the profile indicated inTable 1. The profiles indicated in Table 1 all have roughly similarwavelength dependency, taking dispersion values of −0.5 ps/km/nm or lessin the region called the C-band up to the vicinity of 1570 nm, fromwhich it can be seen that these are optical fibers suitable for a WDM(wavelength division multiplexing) transmission system wherein the Cband is used.

When wavelength dependencies for the chromatic dispersion value asplotted in the graph (b) in FIG. 3 are taken, the range whereinchromatic dispersion values of −0.5 ps/km/nm or less can be taken can beexpanded out to the vicinity of 1600 nm. That is, as compared to opticalfibers having the characteristics represented by the graph (a) in FIG.3, the optical fibers having the characteristics represented by thegraph (b) in FIG. 3 make it possible to expand the range of wavelengthsthat can be used in a WDM transmission system. Profile design examplesthat effect the characteristics represented by the graph (b) in FIG. 3are indicated in Table 2.

TABLE 1 CHROMATIC DISPERSION BENDING Δ1 2 × r2 λcf λop Aeff MFDDISPERSION SLOPE LOSS AT 20φ r2/r1 Δ2/Δ1 [%] [μm] [nm] [nm] [μm²] [μm][ρs/km/nm] [ρs/km/nm²] [dB/m] 8 0.08 0.635 35.23 1238 1490 44.63 7.76−5.69 0.064 4.0 1550 50.06 8.21 −1.96 0.062 11.8 1625 58.13 8.83 2.710.062 35.5 5 0.10 0.660 21.27 1139 1490 44.64 7.76 −6.18 0.071 1.4 155050.12 8.22 −1.95 0.071 4.3 1625 58.09 8.83 3.33 0.071 13.5 10 0.06 0.62044.01 995 1490 44.82 7.78 −5.63 0.063 9.8 1550 50.31 8.23 −1.89 0.06226.9 1625 58.50 8.86 2.69 0.061 75.0 6 0.14 0.655 25.43 1563 1490 48.818.11 −6.59 0.073 1.4 1550 55.30 8.62 −2.20 0.073 4.2 1625 64.81 9.303.33 0.074 12.6 8 0.10 0.615 34.81 1448 1490 48.97 8.12 −6.48 0.067 10.21550 55.47 8.63 −2.50 0.060 26.5 1625 65.22 9.33 2.46 0.066 69.5 10 0.080.600 43.58 1279 1490 49.12 8.14 −5.81 0.066 26.3 1550 55.67 8.65 −1.900.065 64.5 1625 65.56 9.35 2.92 0.064 159.0 6 0.12 0.630 24.99 1346 149051.23 8.30 −7.20 0.076 7.1 1550 58.39 8.84 −2.64 0.076 17.7 1625 68.939.57 3.10 0.077 44.4 7 0.12 0.620 29.93 1519 1490 51.09 8.29 −6.24 0.0717.8 1550 58.16 8.83 −1.98 0.071 19.9 1625 68.72 9.56 3.39 0.072 51.0 100.10 0.600 43.37 1623 1490 51.16 8.30 −6.11 0.067 34.4 1550 58.23 8.84−2.12 0.066 83.0 1625 68.96 9.58 2.83 0.066 202.0

TABLE 2 CHROMATIC DISPERSION BENDING Δ1 2 × r2 λcf λop Aeff MFDDISPERSION SLOPE LOSS AT 20φ r2/r1 Δ2/Δ1 [%] [μm] [nm] [nm] [μm²] [μm][ρs/km/nm] [ρs/km/nm²] [dB/m] 6.5 0.114 0.70 26.13 1425 1490 45.53 7.83−8.36 0.073 2.2 1550 51.81 8.33 −3.99 0.073 6.8 1625 61.16 9.02 1.570.075 20.6 5.0 0.086 0.70 19.32 1004 1490 45.04 7.78 −8.81 0.079 5.41550 51.27 8.29 −4.08 0.079 14.5 1625 60.37 8.96 1.84 0.079 39.0 7.00.086 0.70 28.35 1223 1490 42.99 7.61 −8.60 0.068 2.7 1550 48.73 8.09−4.53 0.068 8.5 1625 57.30 8.75 0.61 0.070 26.5 5.5 0.107 0.75 21.331234 1490 41.70 7.49 −9.62 0.075 0.7 1550 47.27 7.97 −5.10 0.076 2.51625 55.43 8.60 0.63 0.077 8.8 7.0 0.057 0.70 29.06 991 1490 40.06 7.35−7.67 0.064 1.8 1550 44.96 7.78 −3.89 0.063 5.9 1625 52.25 8.37 0.810.063 20.1

FIG. 1B represents a second example of a refractive index distributionshape for a dispersion-shifted optical fiber in this embodiment.

This refractive index distribution shape differs from the refractiveindex distribution shape in the first example described in the foregoingin that the clad 7 has a two-layer structure comprising a first clad 5and a second clad 6.

In this clad 7, the refractive index of the outermost second clad 6 ishigh, while the first clad 5 has a lower refractive index than that ofthe second clad 6.

In this figure, the symbol r3 is the radius of the first clad 5, whileΔ3 is the relative refractive-index difference of the first clad 5 whenthe refractive index of the outermost second clad 6 is taken as thereference. The symbols r1 and r2 are the same as those indicated in FIG.1A, while Δ1 and Δ2, respectively, represent the relativerefractive-index differences of the center core portion 1 and of thestep core portion 2 when the refractive index of the second clad 6 istaken as the reference.

In this example, the center core portion 1 and the step core portion 2are formed from germanium-doped quartz glass, the first clad 5 fromfluorine-doped quartz glass to which has been added fluorine whichexhibits the effect of lowering the refractive index, and the secondclad 6 from pure quartz glass.

As in the first example, moreover, the boundaries between each layer(i.e. the center core portion 1, step core portion 2, first clad 5, andsecond clad 6) need not be definite, but may instead be in a roundedcondition exhibiting so-called sagging.

In dispersion-shifted optical fiber having the refractive indexdistribution shape of the second example, by setting the respectivestructural parameters for the center core portion 1 and the step coreportion 2, namely r1 and Δ1, on one hand, and r2 and Δ2, on the other,so that they fall satisfactorily within the numerical ranges for r2/r1,Δ2/Δ1, and Δ1 indicated in the first example described earlier, and sothat Aeff and the other characteristic values in this embodiment can berealized, the same benefits are obtained as in the first example.

By effecting the configuration with the first clad 5 added, it becomespossible to reduce the bending loss further than with the first example.While not constituting particular limitations, by adopting this secondexample of refractive index distribution shape, the bending loss can beset to 100 dB/m or less, and preferably to 40 dB/m or less.

A benefit can also be realized in that, depending on how the structuralparameters (combinations thereof) are set, moreover, the cutoffwavelength can be made even shorter, or the Aeff can be expandedfurther.

FIG. 4 is a graph that shows the changes in the bending loss resultingfrom combinations of Δ3 and (r3−r2)/r1 when Δ3 and r3 are varied whileholding Δ1, Δ2, r1, and r2 constant. Values of (r3−r2)/r1 are plotted onthe horizontal axis while values of Δ3 are plotted on the vertical axis.Δ1 is 0.61%, Δ2 is 0.05%, and r2/r1 is 10.

From this graph, it is apparent that the bending loss tends to becomesmaller the more Δ3 shifts from zero to a minus value, that is, the morethe refractive index of the first clad 5 becomes smaller and the drop inthe refractive index caused by the first clad 5 becomes greater. Thebending loss also tends to become smaller the more the value of(r3−r2)/r1, which is to say the value of r3, becomes larger.

Thus, because the bending loss varies according to the combinations ofΔ3 and (r3−r2)/r1, there is comparatively great freedom in setting thestructural parameters (Δ3, r3) for the first clad 5 in order tosatisfactorily realize a favorable numerical range for the bending loss.

In FIG. 4, for example, a bending loss of 30 dB/m or so can be obtainedboth with the combination where (r3−r2)/r1 is 0.6 and Δ3 is −0.18%, andwith the combination where (r3−r2)/r1 is 1.8 and Δ3 is −0.05%.Accordingly, if only bending loss is considered, either of thesecombinations may be adopted.

However, because the transmission loss tends to worsen when Δ3 becomessmall (i.e. when that value shifts toward the minus side), it ispreferable that Δ3 be −0.3% or greater.

Also, because problems in manufacturing arise when the value of(r3−r2)/r1 becomes large (i.e. when r3 becomes large), it is preferablethat (r3−r2)/r1 be set at 4.0 or less. And because it is necessary toset Δ3 to a small value when (r3−r2)/r1 is small, transmission losstends to worsen, and problems also arise in manufacturing, wherefore itis preferable that (r3−r2)/r1 be 0.2 or greater.

Table 3 represents specific design examples of dispersion-shiftedoptical fibers that satisfy such conditions as these. In each of theseexamples, characteristics are obtained which satisfy the preferrednumerical ranges for Aeff, dispersion slope, chromatic dispersion value,bending loss, and cutoff wavelength in this embodiment, such as aresuitable for a wavelength division multiplexed transmission system. Thedesign examples in this table are examples that presume applicationsprimarily in the C band. As in the first example, designs are possiblewhich presume specifications for the L band and not only for the C band.

TABLE 3 CHROMATIC DISPERSION BENDING (r3-r2)/ Δ1 Δ3 2 × r3 λcf λop AeffMFD DISPERSION SLOPE LOSS AT 20φ r2/r1 r1 Δ2/Δ1 [%] [%] [μm] [nm] [nm][μm²] [μm] [ρs/km/nm] [ρs/km/nm²] [dB/m] 7 0 0.1 0.625 0 30.41 1346 149047.80 8.03 −5.93 0.067 5.5 1550 54.20 8.52 −1.92 0.067 14.9 1625 63.279.20 3.10 0.068 41.2 7 0.5 0.1 0.625 −0.3 31.96 1235 1490 48.75 8.10−6.69 0.072 4.2 1550 55.25 8.61 −2.40 0.072 10.9 1625 64.83 9.30 3.030.073 28.4 7 1 0.1 0.625 −0.16 34.25 1259 1490 48.51 8.08 −6.17 0.0713.5 1550 54.93 8.59 −1.93 0.071 9.2 1625 64.41 9.27 3.43 0.072 24.3 7 20.1 0.625 −0.16 38.3 1304 1490 48.84 8.11 −6.48 0.072 2.1 1550 55.368.62 −2.16 0.072 5.6 1625 64.98 9.31 3.32 0.074 14.5 10 0 0.09 0.61 043.7 1488 1490 48.55 8.09 −6.22 0.066 19.7 1550 54.94 8.59 −2.33 0.06450.5 1625 64.57 9.29 2.46 0.064 132.5 10 1 0.09 0.61 −0.3 47.99 14961490 48.64 8.10 −5.88 0.066 5.7 1550 55.06 8.60 −1.96 0.065 14.7 162564.71 9.30 2.90 0.065 38.2 10 1.5 0.09 0.61 −0.3 50.17 1567 1490 48.648.10 −6.15 0.066 3.5 1550 55.06 8.60 −2.33 0.065 9.2 1625 64.71 9.302.64 0.065 23.8 10 2 0.09 0.61 −0.06 52.41 1491 1490 48.58 8.09 −6.200.066 10.8 1550 54.98 8.60 −6.20 0.066 27.8 1625 64.62 9.29 2.52 0.06572.5

In this embodiment, a dispersion-shifted optical fiber is obtained whichsatisfies the conditions of being substantially single-mode andexhibiting a bending loss of 100 dB/m or less, wherewith also the Aeffcan be adequately expanded and the dispersion slope sufficientlyreduced. With this embodiment, more particularly, very small values forthe dispersion slope can be realized.

Accordingly, a dispersion-shifted optical fiber can be provided that isparticularly ideal for wavelength division multiplexed transmissionsystems.

Also, because a comparatively simple refractive index distribution shapeis exhibited, there are few structural parameters that need to becontrolled during manufacture, which is a manufacturing advantage,making it possible to obtain the desired characteristics efficiently.

Second Embodiment

A first example of the refractive index distribution shape in thedispersion-shifted optical fiber of this second embodiment is the sameas the refractive index distribution shape diagrammed in FIG. 1A,described earlier, being a dual-shape form configured by a core 4wherein a step core portion 2 is provided about the outer circumferenceof a center core portion 1, and clad 7 of single-layer structure havinga uniform refractive index is provided about the outer circumferencethereof.

The center core portion 1 exhibits the highest refractive index, thestep core portion 2 has a lower refractive index than the center coreportion 1, and the clad 7 has a refractive index lower than the stepcore portion 2.

In this example, the center core portion 1 and step core portion 2 areformed from a germanium-doped quartz glass to which has been addedgermanium which exhibits the effect of raising the refractive index,while the clad 7 is formed from pure quartz glass, for example.

In the refractive index distribution shape of the dispersion-shiftedoptical fiber, furthermore, the boundaries between each layer (i.e. thecenter core portion 1, step core portion 2, and clad 7) need not bedefinite, but may instead be in a rounded condition exhibiting so-calledsagging, and there is no particular limitation thereon so long as thecharacteristics of the dispersion-shifted optical fiber of thisembodiment can be effectively realized.

A second example of the refractive index distribution shape in thedispersion-shifted optical fiber of this embodiment is the same as thatdiagrammed in FIG. 1B described earlier.

This refractive index distribution shape differs from the refractiveindex distribution shape of the first example in that the clad 7 hereexhibits a two-layer structure comprising a first clad 5 provided aboutthe outer circumference of the step core portion 2 (core 4), and asecond clad 6 provided about the outer circumference of the first clad5.

In this example, the first clad 5 is formed from a fluorine-doped quartzglass to which has been added fluorine which acts to lower therefractive index.

The wavelength band used in the dispersion-shifted optical fiber of thisembodiment is selected from a range of 1490 to 1625 nm, but generallyfrom a range of 1490 to 1610 nm, as a wavelength band of suitablewavelength width. For example, a wavelength band (such as 1500 to 1520nm) is selected, from the 1490 to 1530 nm range, that has a prescribedwavelength width. Or a wavelength band (such as 1540 to 1565 nm) isselected, from the 1530 to 1570 nm range, that has a prescribedwavelength width. Or a wavelength band (such as 1570 to 1600 nm) thathas a prescribed wavelength width is selected from the 1570 to 1625 nmrange that is the so-called L band, generally from the 1570 to 1610 nmrange.

Thus one of the characteristics of this embodiment is that thewavelength band used can be selected from the L band.

Alternatively, the entire 1490 to 1625 nm region can be made thewavelength band (transmission wavelength band) used.

In the dispersion-shifted optical fiber of this embodiment, thechromatic dispersion value is made from +0.05 to +10.0 ps/km/nm. Whenthat value is smaller than +0.05 ps/km/nm, the chromatic dispersionvalue approaches zero, and four-wave mixing, which is one of nonlinearoptical effects, readily occurs, wherefore that is problematic. When+10.0 ps/km/nm is exceeded, on the other hand, waveform distortionoccurs, and the transmission characteristics sometimes deterioratemarkedly.

The Aeff is found by the same mathematical formula as was indicated inthe first embodiment described earlier.

The dispersion-shifted optical fiber in this embodiment has an Aeff of45 to 70 μm² in the wavelength band used, wherefore the nonlinearoptical effects can be suppressed. When the Aeff is less than 45 μm²,the reduction in the nonlinear optical effects is insufficient, whereaswhen 70 μm² is exceeded, manufacture becomes very difficult.

The very small values of 0.050 to 0.075 ps/km/nm² can be realized forthe dispersion slope in the wavelength band used. As a consequence,deterioration in transmission induced by the dispersion slope inwavelength division multiplexed transmissions can be prevented.

The bending loss is defined in the same way that it was earliermentioned.

The smaller the bending loss is the better. In this embodiment, thebending loss is made to be 100 dB/m or less, and preferably 50 dB/m orless. When 100 dB/m is exceeded, transmission loss readily worsens dueto slight bends imparted to the dispersion-shifted optical fiber, andunnecessary losses occur during laying operations or other handling,which is problematic.

Also, because the dispersion-shifted optical fiber in this embodiment issingle-mode optical fiber, it is necessary to have a cutoff wavelengththat guarantees substantially single-mode propagation in the wavelengthband used.

As was stated earlier, ordinary cutoff wavelengths are defined by valuesbased on the CCITT's 2m method (hereinafter called the 2m method).However, in actual long-length use conditions, single-mode propagationis possible even when this value is on the longer wavelength side fromthe lower limiting value in the wavelength band used.

That being so, in the dispersion-shifted optical fiber of thisembodiment, the cutoff wavelength defined by the 2m method is set sothat single-mode propagation is possible according to the length of thedispersion-shifted optical fiber used and the wavelength band used. Morespecifically, if the cutoff wavelength obtained with the 2m method is1800 nm, under the large length conditions of 5000 m or so, it ispossible to effect single-mode propagation in the wavelength band usedas described in the foregoing.

In this embodiment, moreover, the larger diameter solution is used forthe core diameter (r2×2). More specifically, as will be describedsubsequently, in setting the four structural parameters r2, r1, Δ2, andΔ1 in the refractive index distribution shape diagrammed in FIG. 1A, andin setting the six structural parameters that include those noted aboveplus r3 and Δ3 in the refractive index distribution shape diagrammed inFIG. 1B, design conditions are established such that the core diameterbecomes the larger diameter solution, and such that such characteristicvalues as Aeff and dispersion slope are satisfied in the desiredwavelength band described in the foregoing. Also, such conventionalmethods as CVD and VAD can be employed as the actual method ofmanufacturing the dispersion-shifted optical fiber of this embodiment.Because the refractive index distribution shape for thedispersion-shifted optical fiber in this embodiment is in a three-layeror four-layer configuration, and because it is a comparatively simplestep shape, controlling the structural parameters is comparatively easy.

FIG. 5 is a graph representing an example of the results of an analysisof the structural parameters of a dispersion-shifted optical fiberhaving the refractive index distribution shape diagrammed in FIG. 1A.

The values 5, 7, and 10 corresponding to the symbols ⋄, □, and Δrepresented in the graph are values for r2/r1 that is the ratio betweenthe radiuses of the center core portion 1 and the step core portion 2diagrammed in FIG. 1A. Aeff is plotted on the horizontal axis anddispersion slope on the vertical axis, both of which are values for thewavelength 1550 nm.

It will be seen from this graph that the larger the value of r2/r1becomes, the more can the dispersion slope be reduced. In order toobtain bending loss and Aeff values within the preferable numericalranges noted in the foregoing, r2/r1 should be set at 4 or higher. Whenthat value is less than 4, it is very difficult to obtain goodcharacteristics. When the value set exceeds 12, on the other hand,manufacturability declines. Hence the real upper limit is considered tobe 12. These conditions are the same in the refractive indexdistribution shape diagrammed in FIG. 1B.

In this embodiment, furthermore, Δ1 is made to range from 0.55% to 0.75%in the refractive index distribution shapes diagrammed in FIGS. 1A and1B. When Δ1 is less than 0.55%, it becomes very difficult to set thechromatic dispersion value in a desirable range, and bending loss tendsto become great. If Δ1 exceeds 0.75%, it becomes very difficult to makeAeff sufficiently large.

It is preferable that the Δ2/Δ1 ratio be from 0.05 to 0.15. At valueslower than 0.05, bending loss becomes large, so that is problematic.When 0.15 is exceeded, the dispersion slope will exceed the definedrange, which is problematic for use in wavelength division multiplexedtransmission.

Table 4 reflects simulation results representing structural parametersand characteristic values for specific design examples ofdispersion-shifted optical fibers having the refractive indexdistribution shape diagrammed in FIG. 1A which satisfy these conditions.

TABLE 4 CHROMATIC DISPERSION BENDING Δ1 r2 λc Aeff MFD DISPERSION SLOPELOSS AT 20φ r2/r1 Δ2/Δ1 [%] [μm] [μm] [μm²] [μm] [ρs/km/nm] [ρs/km/nm²][dB/m] 10.00 0.07 0.57 25.66 1.09 51.55 8.34 5.35 0.057 17.70 10.00 0.070.58 25.78 1.10 50.24 8.23 5.51 0.057 10.30 8.00 0.08 0.60 20.10 1.3049.71 8.19 4.61 0.057 5.66 5.00 0.12 0.60 12.60 1.32 51.85 8.36 5.000.061 2.19 4.00 0.15 0.62 9.54 1.28 52.89 8.45 4.68 0.068 1.69 4.00 0.150.65 9.63 1.35 49.56 8.18 4.53 0.066 0.35 MFD is the mode field diameterλc (cutoff wavelength) is based on the 2m method The estimatedwavelength is 1550 nm Bending loss is measured for φ20 mm

In every example, preferable numerical ranges are satisfactorilyrealized for the Aeff, dispersion slope, chromatic dispersion value,bending loss, and cutoff wavelength in this embodiment, andcharacteristics are obtained which are suitable for wavelength divisionmultiplexed transmission systems.

Furthermore, these characteristic values will not necessarily beobtained, even when suitable values are selected and combined from thenumerical ranges of such structural parameters as described in theforegoing; it is necessary to select combinations of structuralparameters that satisfy the characteristic values from the graph(s) andsimulation results described in the foregoing. That being so, because itis very difficult to specify the dispersion-shifted optical fiber ofthis embodiment by the structural parameters, that specification is madehere by the characteristic values.

In the refractive index distribution shape diagrammed in FIG. 1B,moreover, Δ3 and r3 are set. By giving the clad 7 a two-layer structurecomprising the first clad 5 and the second clad 6, the cutoff wavelengthcan be made even shorter than in the first example, by combining(setting) the structural parameters, and the Aeff can be furtherexpanded.

FIGS. 6A and 6B represent the relationship between Δ3 and bending lossand the relationship between Δ3 and Aeff, respectively, in a refractiveindex distribution shape comprising the two-layer clad structurediagrammed in FIG. 1B. The estimated wavelength is 1550 nm.

Δ1, Δ2, r1, and r2 are all common values and are set constant. That is,Δ1 is 0.56%, and Δ2 is 0.06%.

From these graphs it will be seen that Aeff can be made larger when Δ3becomes smaller, but the bending loss then also becomes large.

The behavior will also be different according to the value of(r3−r2)/r1.

Accordingly, structural parameters are set so as to satisfy thepreferable numerical ranges of the characteristics described earlier,taking the relationships between these structural parameters and thecharacteristics into consideration.

It is preferable to make the value of Δ3−0.1% or greater. The reason forthis is that, when Δ3 is smaller than −0.1%, the transmissioncharacteristics will sometimes deteriorate, depending on thecombinations with the other structural parameters.

It is also preferable to make the value of (r3−r2)/r1 4.0 or less formanufacturing reasons. When (r3−r2)/r1 is small, however, it becomesnecessary to make Δ3 small. Therefore, in order to limit deteriorationin the transmission characteristics, as described earlier, (r3−r2)/r1should be made 0.2 or greater.

In Table 5 are given simulation results indicating structural parametersand characteristic values in specific design examples fordispersion-shifted optical fibers that satisfy these conditions.

In every example, preferable numerical ranges are satisfactorilyrealized for the Aeff, dispersion slope, chromatic dispersion value,bending loss, and cutoff wavelength in the dispersion-shifted opticalfiber of this embodiment, and characteristics are obtained which aresuitable for wavelength division multiplexed transmission systems.

TABLE 5 CHROMATIC DISPERSION BENDING (r3-r2)/ Δ1 Δ3 r3 λc Aeff MFDDISPERSION SLOPE LOSS AT 20φ r2/r1 r1 Δ2/Δ1 [%] [%] [μm] [μm] [μm²] [μm][ρs/km/nm] [ρs/km/nm²] [dB/m] 10.00 4.00 0.07 0.57 −0.05 35.80 1.2951.66 8.35 5.23 0.058 7.04 5.00 1.50 0.12 0.60 −0.05 16.00 1.26 52.538.42 4.59 0.064 2.60 8.00 4.00 0.08 0.60 −0.03 30.08 1.36 49.78 8.194.52 0.057 3.77 10.00 2.00 0.07 0.58 −0.03 30.98 1.14 50.21 8.22 5.550.057 7.69 4.00 2.50 0.15 0.62 −0.05 16.22 1.30 51.21 8.32 5.95 0.0660.36 4.00 2.50 0.15 0.65 −0.02 15.60 1.32 49.53 8.18 4.65 0.067 0.324.00 0.60 0.11 0.56 −0.1 11.23 1.07 56.0 8.70 5.96 0.068 11.70 4.00 0.600.11 0.56 −0.08 11.15 1.07 56.4 8.73 5.73 0.068 13.30 MFD is the modefield diameter λc (cutoff wavelength) is based on the 2m method Theestimated wavelength is 1550 nm Bending loss is measured for φ20 mm

The estimated wavelength for the dispersion-shifted optical fibercharacteristics given in Tables 4 and 5 is 1550 nm.

The results of the same simulations conducted with the estimatedwavelength set at 1610 nm are given in Tables 6 and 7. Characteristicvalues that satisfy the numerical ranges in this embodiment are obtainedin all of the examples given in Tables 4 to 7. That being so, with allof the dispersion-shifted optical fibers represented in Tables 4 to 7,not only in the 1550 nm band, but also in a broader band (1490 to 1610nm, for example) to which 1570 to 1625 have been added, chromaticdispersion can be made small, bending loss made small, and single-modetransmission guaranteed, while at the same time, due to the expansion inAeff, the nonlinear optical effects can be suppressed, and, due to thesmall dispersion slope, transmission deterioration in wavelengthdivision multiplexed transmissions can be limited.

Accordingly, transmission characteristic enhancement can be effectedeven in a wavelength division multiplexed transmission system employedin a wide wavelength band to which the L band has been added.

TABLE 6 CHROMATIC DISPERSION BENDING Δ1 r2 λc Aeff MFD DISPERSION SLOPELOSS AT 20φ r2/r1 Δ2/Δ1 [%] [μm] [μm] [μm²] [μm] [ρs/km/nm] [ρs/km/nm²][dB/m] 10 0.07 0.57 25.7 1.09 56.8 8.75 8.73 0.056 44.3 5 0.12 0.6 12.61.32 57.2 8.79 8.62 0.060 6.06 8 0.08 0.6 20.1 1.30 54.8 8.6 7.98 0.05515.6 10 0.07 0.58 25.8 1.10 55.2 8.63 8.85 0.055 27.4 4 0.15 0.62 9.51.28 58.5 8.89 8.73 0.067 4.69 4 0.15 0.65 9.6 1.35 54.6 8.59 8.43 0.0651.18 MFD is the mode field diameter λc (cutoff wavelength) is based onthe 2m method The estimated wavelength is 1610 nm Bending loss ismeasured for φ20 mm

TABLE 7 CHROMATIC DISPERSION BENDING (r3-r2)/ Δ1 Δ3 r3 λc Aeff MFDDISPERSION SLOPE LOSS AT 20φ r2/r1 r1 Δ2/Δ1 [%] [%] [μm] [μm] [μm²] [μm][ρs/km/nm] [ρs/km/nm²] [dB/m] 10 4 0.07 0.57 −0.05 35.80 1.285 56.948.77 8.62 0.056 17.7 5 1.5 0.12 0.6 −0.05 16.00 1.257 58.1 8.86 8.380.063 6.87 8 4 0.08 0.6 −0.03 30.08 1.358 54.88 8.61 7.91 0.056 10.3 102 0.07 0.58 −0.03 30.98 1.14 55.14 8.62 8.90 0.055 20.5 4 2.5 0.15 0.62−0.05 16.22 1.3 56.15 8.71 9.87 0.065 1.09 4 2.5 0.15 0.65 −0.02 15.601.316 54.48 8.58 8.62 0.066 1 4 0.6 0.11 0.56 −0.1 11.23 1.071 61.879.14 10.00 0.067 26 4 0.6 0.11 0.56 −0.08 11.15 1.066 62.44 9.19 9.790.067 29.2 MFD is the mode field diameter λc (cutoff wavelength) isbased on the 2m method The estimated wavelength is 1610 nm Bending lossis measured for φ20 mm

Benefits such as those described below are realized in thedispersion-shifted optical fiber of this embodiment.

That is, under such conditions as that of being substantiallysingle-mode and maintaining bending loss at 100 dB/m or less, both thesuppression of nonlinear optical effects due to Aeff expansion and thereduction of the dispersion slope can be satisfactorily realizedsimultaneously, and good transmission characteristics obtained. Thesecharacteristics are particularly effective in wavelength divisionmultiplexed transmission.

Also, because of the comparatively simple step structure, it is easy tocontrol the structural parameters during manufacture, anddispersion-shifted optical fiber having stable characteristics can bemanufactured efficiently.

Also, the characteristics noted above can be maintained even in a broadwavelength band that extends from 1490 to 1625 nm, with the L bandincluded, and it is possible to cope with the longer haul covered andhigher volumes handled by wavelength division multiplexed systems.

In particular, practicable bending loss can be realized in the L band.

Third Embodiment

One example of the refractive index distribution shape in thedispersion-shifted optical fiber in this embodiment is the same as thatdiagrammed in FIG. 1A described earlier, comprising a core 4 having astep core portion 2 provided about the outer circumference of a centercore portion 1, and a one-layer structured clad 7 having uniformrefractive index provided about the outer circumference thereof.

The center core portion 1 has the highest refractive index, the stepcore portion 2 has a lower refractive index than the center core portion1, and the clad 7 has a lower refractive index than the step coreportion 2.

In this example, the center core portion 1 and the step core portion 2are formed from germanium-doped quartz glass to which has been addedgermanium which exhibits an effect to raise the refractive index, whilethe clad 7 is formed from pure quartz glass.

In the refractive index distribution shape of the dispersion-shiftedoptical fiber, furthermore, the boundary between each layer (i.e. thecenter core portion 1, step core portion 2, and clad 7) need not bedefinite, but may instead be in a rounded condition exhibiting so-calledsagging, and need not be particularly limited so long as thecharacteristics of the dispersion-shifted optical fiber of thisembodiment are effectively exhibited.

For the wavelength band used in the dispersion-shifted optical fiber ofthis embodiment, a wavelength band of suitable wavelength width isselected from the range of 1490 to 1625 nm. Depending on theamplification wavelength band based on the optical fiber amplifier usedin the light communication system, for example, a wavelength band (suchas 1500 to 1520 nm, for example) that has a prescribed wavelength widthis selected from the range of 1490 to 1530 nm. Or a wavelength band(such as 1540 to 1565 nm, for example) that has a prescribed wavelengthwidth is selected from the range of 1530 to 1570 nm. Or a wavelengthband (such as 1570 to 1600 nm, for example) that has a prescribedwavelength width is selected from the range of 1570 to 1625 nm. Ofthese, the 1530 to 1570 nm range is the one being most used in recentyears.

The Aeff is found by the same formula as was indicated in the firstembodiment.

In this embodiment, because the Aeff ranges from 65 to 95 μm² in thewavelength band used, it is possible to suppress the nonlinear opticaleffects. When 95 μm² is exceeded, manufacture becomes very difficult.

The dispersion slope in the wavelength band used is made to be from 0.08to 0.14 ps/km/nm². If within this range, in wavelength divisionmultiplexed transmission, large transmission deterioration induced bythe dispersion slope can be prevented.

Bending loss is defined in the same way as noted earlier.

The smaller the bending loss the better. In this embodiment, bendingloss is made 100 dB/m or less, and preferably 50 dB/m or less. When 100dB/m is exceeded, transmission loss readily worsens due to slight bendsimparted to the dispersion-shifted optical fiber, and unnecessary lossesoccur during laying operations or other handling, which is problematic.

The absolute values of the chromatic dispersion values are made to rangefrom 0.5 to 8.0 ps/km/nm. When the absolute value is smaller than 0.5ps/km/nm, the chromatic dispersion value approaches zero, which isproblematic because four-wave mixing, which is one nonlinear opticaleffect, then readily occurs. When 8.0 ps/km/nm is exceeded, on the otherhand, waveform distortion occurs, and transmission characteristicdeterioration may become great.

As will be described later in more specific terms, furthermore, becausethe chromatic dispersion value can be controlled so as to be eitherpositive or negative, various light communication system demands can becoped with, and dispersion-shifted optical fiber can be designed whichis capable of being employed in soliton transmission, for example.

Furthermore, because the dispersion-shifted optical fiber in thisembodiment is single-mode optical fiber, it is necessary to have acutoff wavelength that guarantees substantially single-mode propagationin the wavelength band used.

Ordinary cutoff wavelengths are defined by values based on the CCITT's2m method (hereinafter called the 2m method). However, under actuallong-length use conditions, single-mode propagation is possible evenwhen this value is on the longer wavelength side from the lower limitingvalue in the wavelength band used.

That being so, in the dispersion-shifted optical fiber of thisembodiment, the cutoff wavelength defined by the 2m method is set sothat single-mode propagation is possible according to the length of thedispersion-shifted optical fiber used and the wavelength band used. Morespecifically, if the cutoff wavelength in the 2m method is 1800 nm,under large length conditions of about 5000 m or larger, it is possibleto effect single-mode propagation in the wavelength band used asdescribed in the foregoing.

In this embodiment, moreover, the smaller diameter solution is used forthe core diameter as described in the foregoing. More specifically, insetting the four structural parameters r2, r1, Δ2, and Δ1, designconditions are established such that the core diameter becomes thesmaller diameter solution, and such that such characteristic values asAeff and dispersion slope are satisfied in the desired wavelength banddescribed in the foregoing. Also, such conventional methods as CVD andVAD can be employed as the actual method of manufacturing thedispersion-shifted optical fiber of this embodiment.

FIG. 7 is a graph representing the results of an analysis of thestructural parameters of a dispersion-shifted optical fiber, showing thetrack of the smaller diameter solution when Δ2/Δ1 and Δ1 are varied whenthe value of r2/r1 is 5.0, 7.0, and 9.0, respectively.

The Δ2/Δ1 curves represent the characteristics when Δ1 is varied whileholding Δ2/Δ1 constant at the values indicated on the curves. The Δ1curves, on the other hand, represent the characteristics when Δ2/Δ1 isvaried while holding Δ1 constant at the values indicated on the curves.

When r2/r1 is 9.0, for example, when moving from right to left in thegraph on the Δ2/Δ1=0.14 curve, Δ1 changes from 0.9 to 2.0. Then, forexample, the point where the Δ2/Δ1=0.14 curve intersects the Δ1=1.4curve represents the characteristics of the dispersion-shifted opticalfiber when Δ2/Δ1 is made 0.14 and Δ1 is made 1.4.

The analysis conditions, moreover, are that the wavelength used is 1550nm and the chromatic dispersion value at the wavelength used is −2.0ps/km/nm. The zero dispersion wavelength, while not constant because thedispersion slope differs, is nevertheless roughly 1565 nm or higher, onthe long wavelength side from the wavelength (band) used.

FIGS. 8 and 9 are graphs, like the graph in FIG. 7, representing thedistribution of characteristic values associated with changes in Δ2/Δ1and Δ1 when r2/r1 is 7.0 and 9.0, respectively. In these graphs, thecutoff wavelength (λc) and dispersion slope distribution are alsoindicated.

That is, the cutoff wavelength distribution is also indicated on thecurves for which Δ2/Δ1 is 0.10, 0.12, 0.14, and 0.16. When Δ2/Δ1 is0.10, for example, the cutoff wavelength is distributed in a range of1.0 to 1.1. When Δ2/Δ1 is 0.12, on the other hand, the cutoff wavelengthis distributed in ranges of 1.1 to 1.2 and 1.2 to 1.3. And, as can beseen, when Δ2/Δ1 is constant, the cutoff wavelength becomes shorter whenΔ1 is made large.

The dispersion slope curves have an inverted U shape, distributed in acontoured pattern. The farther to the outside of this contoured patterndistribution, the smaller the dispersion slope, and the closer inside,the larger.

Accordingly, in the graph given in FIG. 8, for example, when Δ2/Δ1 is0.14, and Δ1 is 1.4 (the point where the Δ2/Δ1=0.14 curve intersects theΔ1=1.4 curve), a dispersion-shifted optical fiber is obtained whereinthe cutoff wavelength is in the range of 1300 to 1400 nm, and thedispersion slope is in the range of 0.122 to 0.124 ps/km/nm².

From the graph given in FIG. 8 it will be seen that, by setting r2/r1 at5 times or greater, a practicable bending loss is realized in the regionwhere Aeff is 65 μm² or greater.

When r2/r1 is large, on the other hand, a larger Aeff can be obtained.However, as may be seen by comparing the graphs given in FIG. 8 and FIG.9, the dispersion slope tends to become large when r2/r1 is large. Inorder to obtain dispersion-shifted optical fiber suited to a wavelengthdivision multiplexed system, it is preferable that the dispersion slopein the wavelength band used be 0.14 ps/km/nm² or less, and, to that end,r2/r1 is made 10 or less.

Accordingly, when provision is made so that r2/r1=x, a range of 5≦x≦10is desirable.

Also, when Δ2/Δ1 is too small, the bending loss becomes large, becomingpractically unusable, wherefore Δ2/Δ1 is made to be 0.08 or larger. IfΔ2/Δ1 is too large, on the other hand, the cutoff wavelength becomeslong, and single-mode propagation can no longer be secured in thewavelength band used, wherefore Δ2/Δ1 is made to be 0.22 or less.

Accordingly, when provision is made so that Δ2/Δ1=y, a range of0.08≦y≦0.22 is desirable.

The value of Δ2/Δ1 (y) can also be adjusted according to the bendingloss allowed and the cutoff wavelength required in different individuallight communication systems.

Δ1 is made to range from 0.6 to 1.2%. When this value is less than 0.6%,bending loss becomes too large, and, in some cases, the chromaticdispersion value cannot be controlled to the desired value. When 1.2% isexceeded, Aeff cannot be sufficiently expanded, and, in some cases,Rayleigh loss becomes large.

The preferable ranges for these factors, namely r2/r1 (x), Δ2/Δ1 (y),and Δ1, are the same in cases where the zero dispersion wavelength is onthe short wavelength side from the wavelength band used.

Design is effected so that combinations of structural parameters areselected from these numerical ranges which satisfy the characteristicsof the dispersion-shifted optical fiber of this embodiment.

In the dispersion-shifted optical fiber of this embodiment, furthermore,r2, that is, the radius of the core, is not particularly limited, butthe range thereof is ordinarily 4 to 12 μm. And the outer diameter ofthe clad 7 (i.e. of the dispersion-shifted optical fiber) is ordinarilymade approximately 125 μm.

In the dispersion-shifted optical fiber of this embodiment, moreover,the limits of the structural parameters will differ according as towhether the fiber has the zero dispersion wavelength on the longwavelength side from the wavelength band used, or on the shortwavelength side thereof.

When the zero dispersion wavelength is on the long wavelength side fromthe wavelength band used, the limitations noted below are given.

Specifically, in order to obtain dispersion-shifted optical fiber withan Aeff of 65 to 75 μm² and a dispersion slope of 0.125 ps/km/nm² orless, the conditions noted below should be satisfied.

When r2/r1 is expressed by x and Δ2/Δ1 by y,

6≦x≦7,

0.1≦y≦0.18,

y≧(−0.02x+0.24), and

0.6%≦Δ1≦1.2%.

And in order to obtain dispersion-shifted optical fiber with an Aeff of70 to 80 μm² and a dispersion slope of 0.130 ps/km/nm² or less, theconditions noted below should be satisfied.

7≦x≦8,

0.1≦y≦0.16,

y≧(−0.016x+0.21), and

0.6%≦Δ1≦1.2%.

And in order to obtain dispersion-shifted optical fiber with an Aeff of75 to 85 μm² and a dispersion slope of 0.135 ps/km/nm² or less, theconditions noted below should be satisfied.

7≦x≦8.5,

0.1≦y≦0.16,

(−0.02x+0.26)≦y≦(−0.02x+0.32), and

0.6%≦Δ1≦1.2%.

When there is a zero dispersion wavelength on the short wavelength sidefrom the wavelength band used, on the other hand, the limitations notedbelow are given.

Specifically, in order to obtain dispersion-shifted optical fiber withan Aeff of 65 to 75 μm² and a dispersion slope of 0.110 ps/km/nm² orless, the conditions noted below should be satisfied.

5≦x≦8,

0.12≦y≦0.22,

(−0.02x+0.24)≦y≦(−0.02x+0.34), and

0.6%≦Δ1≦1.2%.

Here, when x is large and y is small, even if x (r2/r1) and y (Δ2/Δ1)satisfy the ranges noted, it is necessary to set the value of Δ1 large.As a consequence, there is a possibility that transmission loss willworsen due to the increase in Rayleigh loss.

In order to prevent that from happening, Δ1 is limited. That is, when Δ1is set within the range noted above, the resulting transmission losspresents no problem in practice. The same reason applies to thelimitation on Δ1 noted below.

In order to obtain dispersion-shifted optical fiber with an Aeff of 70to 80 μm² and a dispersion slope of 0.115 ps/km/nm² or less, theconditions noted below should be satisfied.

5.5≦x≦8,

0.12 <y≦0.20,

(−0.02x+0.25)≦y≦(−0.02x+0.33), and

0.6%≦Δ1≦1.2%.

In order to obtain dispersion-shifted optical fiber with an Aeff of 75to 85 μm² and a dispersion slope of 0.125 ps/km/nm² or less, theconditions noted below should be satisfied.

6≦x≦8,

0.12≦y≦0.20,

(−0.02x+0.26)≦y≦(−0.02x+0.35), and

0.6%≦Δ1≦1.2%.

It goes without saying that the specific numerical values of r1, r2, Δ1,and Δ2 must be adjusted further, within the ranges noted above,depending on such factors as the wavelength band used and the settingconditions for the zero dispersion wavelength.

In the following, design examples are cited and specifically described.

Tables 8 and 9 give structural parameters and characteristic values fordispersion-shifted optical fibers prototyped with the CVD method, usingthe smaller diameter solution. In the tables, λc is the cutoffwavelength and MFD is the mode field diameter.

TABLE 8 CHROMATIC DISPERSION SENDING Δ1 λc Aeff MFD DISPERSION SLOPELOSS AT 20φ NO. r2/r1 Δ2/Δ1 [%] [μm] [μm²] [μm] [ρs/km/nm] [ρs/km/nm²][dB/m] 1 6.0 0.12 1.10 1.10 68.8 9.35 −2.1 0.116 39.3 2 7.0 0.16 1.101.57 70.6 9.43 −2.3 0.124 3.0 3 6.5 0.14 1.00 1.34 71.4 9.51 −1.7 0.12015.8 4 7.0 0.14 0.95 1.44 76.5 8.85 −2.2 0.122 17.5 5 7.0 0.16 0.85 1.6377.0 9.89 −2.1 0.117 16.6 6 7.5 0.11 1.10 1.32 76.0 9.71 −1.8 0.128 26.87 7.5 0.15 0.70 1.60 77.7 10.01 −1.9 0.105 56.5 8 8.0 0.12 1.20 1.4079.6 9.90 −2.0 0.131 26.4 9 8.5 0.12 1.20 1.48 83.3 10.08 −2.3 0.13326.7 * All characteristic values are values for 1550 nm

TABLE 9 CHROMATIC DISPERSION BENDING Δ1 λc Aeff MFD DISPERSION SLOPELOSS AT 20φ NO. r2/r1 Δ2/Δ1 [%] [μm] [μm²] [μm] [ρs/km/nm] [ρs/km/nm²][dB/m] 10 5.0 0.18 0.80 1.26 71.3 9.65 2.2 0.099 14.7 11 6.0 0.16 1.101.24 71.1 9.54 2.1 0.104 10.6 12 6.5 0.17 1.20 1.39 69.9 9.43 1.8 0.1072.3 13 6.0 0.18 0.90 1.44 76.8 9.92 2.3 0.106 8.7 14 6.0 0.14 1.10 1.1274.9 9.79 2.1 0.102 44.5 15 7.0 0.16 1.20 1.43 75.2 9.74 1.9 0.111 4.416 6.0 0.18 0.80 1.47 81.1 10.20 2.4 0.105 17.4 17 7.0 0.14 1.15 1.3080.0 10.03 2.0 0.110 23.4 18 7.5 0.16 1.10 1.56 83.5 10.20 2.1 0.1158.1 * All characteristic values are values for 1550 nm

The dispersion-shifted optical fibers numbered 1 to 9 in Table 8 aredesigned such that, at 1550 nm, the chromatic dispersion value isnegative, in the vicinity of −2 ps/km/nm, with the zero dispersionwavelength approximately 1565 nm or greater, and the zero dispersionwavelength on the long wavelength side from the wavelength band used.

No. 1 to 3 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 70 μm². All of these dispersion-shiftedoptical fibers satisfy the preferred structural parameter conditionsnoted earlier. A dispersion slope value of 0.125 ps/km/nm² or less isobtained.

No. 4 to 6 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 75 μm². With these, a dispersion slope valueof 0.130 ps/km/nm² or less is obtained.

No. 7 to 9 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 80 μm². With these, a dispersion slope valueof 0.135 ps/km/nm² or less is obtained.

No. 10 to 18 in Table 9 are designed such that, at 1550 nm, thechromatic dispersion value is positive, in the vicinity of 2 ps/km/nm,the zero dispersion wavelength is approximately 1540 nm or less, and thezero dispersion wavelength is on the short wavelength side from thewavelength (band) used.

No. 10 to 12 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 70 μm². With these, a dispersion slope valueof 0.110 ps/km/nm² or less is obtained.

No. 13 to 15 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 75 μm². With these, a dispersion slope valueof 0.115 ps/km/nm² or less is obtained.

No. 16 to 18 are dispersion-shifted optical fiber design examples havingan Aeff in the vicinity of 80 With these, a dispersion slope value of0.125 ps/km/nm² or less is obtained.

In this embodiment, the chromatic dispersion value for the wavelengthband used is controlled within a certain range, never becoming zero, andthe Aeff is expanded. Therefore nonlinear optical effects are notreadily produced, making it possible to provide dispersion-shiftedoptical fiber suitable for long haul systems such as light amplifyingrelay transmission systems wherein optical fiber amplifiers are used.The dispersion slope is controlled so as to be small, moreover, andapplication in wavelength division multiplexed transmission is possible.

Because the chromatic dispersion value can be adjusted to either apositive or negative value, moreover, the sign of the chromaticdispersion value can be set according to the light communication system.

INDUSTRIAL APPLICABILITY

Based on the present invention, a dispersion-shifted optical fiber isobtained wherewith, under conditions such that the dispersion-shiftedoptical fiber is substantially single mode, and such that bending lossis held down to 100 dB/m or lower, both Aeff expansion and dispersionslope reduction can be satisfactorily effected.

The structure of the dispersion-shifted optical fibers of the presentinvention is simple, furthermore, wherefore dispersion-shifted opticalfiber that exhibits stable characteristics can be efficientlymanufactured.

What is claimed is:
 1. A dispersion-shifted optical fiber hang arefractive index distribution shape, comprising a center core portion ofhigh refractive index, a step core portion of lower refractive indexthan said center core portion, provided about outer circumferencethereof; and clad of lower refractive index than said step core portion,provided about outer circumference of the step core portion; wherein thedispersion-shifted optical fiber has, in a used wavelength band selectedfrom 1490 to 1625 nm, Aeff of 45 to 90 μm², dispersion slope of from0.05 to 0.14 ps/km/nm², bending loss of 100 dB/m or less, and chromaticdispersion value of either from −0.5 to −8.0 ps/km/nm or from +0.05 to+10.0 ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; wherein larger diameter solution isadopted for core diameter, and said dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45to 70 μm², dispersion slope of from 0.05 to 0.08 ps/km/nm², bending lossof 100 dB/m or less, and chromatic dispersion value of from −0.5 to −8.0ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; and wherein when radius of saidcenter core portion is represented as r1, radius of said step coreportion as r2, relative refractive-index difference of said center coreportion when refractive index of outermost clad is taken as reference asΔ1, and relative refractive-index difference of said step core portionas Δ2, r2/r1 is from 4 to 12, Δ2/Δ1 is from 0.05 to 0.15, and Δ1 is from0.55 to 0.85%.
 2. A dispersion-shifted optical fiber having a refractiveindex distribution shape, comprising a center core portion of highrefractive index, a step core portion of lower refractive index thansaid center core portion, provided about outer circumference thereof;and clad of lower refractive index than said step core portion, providedabout outer circumference of the step core portion; wherein thedispersion-shifted optical fiber has, in a used wavelength band selectedfrom 1490 to 1625 nm, Aeff of 45 to 90 μm², dispersion slope of from0.05 to 0.14 ps/km/nm², bending loss of 100 dB/m or less, and chromaticdispersion value of either from −0.5 to −8.0 ps/km/nm or from +0.05 to+10.0 ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; wherein larger diameter solution isadopted for core diameter, and said dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45to 70 μm², dispersion slope of from 0.05 to 0.08 ps/km/nm², bending lossof 100 dB/m or less, and chromatic dispersion value of from −0.5 to −8.0ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; and wherein said clad comprisesfirst clad provided about outer circumference of said step core portionand second clad having a higher refractive index than said first clad,provided about outer circumference of the first clad.
 3. Thedispersion-shifted optical fiber according to claim 2, wherein whenradius of said center core portion is represented as r1, radius of saidstep core portion as r2, radius of the first clad as r3, relativerefractive-index difference of said center core portion when refractiveindex of said outermost clad is taken as reference as Δ1, relativerefractive-index difference of said step core portion as Δ2, andrelative refractive-index difference of said first clad as Δ3, r2/r1 isfrom 4 to 12, Δ2/Δ1 is from 0.05 to 0.15, Δ1 is from 0.55 to 0.85%, Δ3is from −0.3 to 0%, and (r3−r2)/r1 is from 0.2 to 4.0.
 4. Adispersion-shifted optical fiber having a refractive index distributionshape, comprising a center core portion of high refractive index, a stepcore portion of lower refractive index than said center core portion,provided about outer circumference thereof; and clad of lower refractiveindex than said step core portion, provided about outer circumference ofthe step core portion; wherein the dispersion-shifted optical fiber has,in a used wavelength band selected from 1490 to 1625 nm, Aeff of 45 to90 μm², dispersion slope of from 0.05 to 0.14 ps/km/nm², bending loss of100 dB/m or less, and chromatic dispersion value of either from −0.5 to−8.0 ps/km/nm or from +0.05 to +10.0 ps/km/nm, and has a cutoffwavelength such that substantially single-mode propagation is realized;wherein larger diameter solution is adopted for core diameter, saiddispersion-shifted optical fiber has, in a used wavelength band selectedfrom 1490 to 1625 nm, Aeff of 45 to 70 μm², dispersion slope of from0.05 to 0.075 ps/km/nm²; bending loss of 100 dB/m or less, and chromaticdispersion value of from +0.05 to +10.0 ps/km/nm, and has a cutoffwavelength such that substantially single-mode propagation is realized;and wherein, when radius of said center core portion is represented asr1, radius of said step core portion as r2, relative refractive-indexdifference of said center core portion when refractive index of saidoutermost clad is taken as reference as Δ1, and relativerefractive-index difference of said step core portion as Δ2, r2/r1 isfrom 4 to 12, Δ1 is from 0.55 to 0.75%, and Δ2/Δ1 is from 0.05 to 0.15.5. The dispersion-shifted optical fiber according to claim 4, whereinsaid clad comprises first clad provided about outer circumference ofsaid step core portion and second clad provided about outercircumference thereof.
 6. The dispersion-shifted optical fiber accordingto claim 4, wherein when radius of said center core portion isrepresented as r1, radius of said step core portion as r2, radius ofsaid first clad as r3, relative refractive-index difference of saidcenter core portion when refractive index of said second clad is takenas reference as Δ1, relative refractive-index difference of said stepcore portion as Δ2, and relative refractive-index difference of saidfirst clad as Δ3, r2/r1 is from 4 to 12, Δ1 is from 0.55 to 0.75%, Δ2/Δ1is from 0.05 to 0.15, Δ3 is from −0.1 to 0%, and (r3−r2)/r1 is from 0.2to 4.0.
 7. A dispersion-shifted optical fiber having a refractive indexdistribution shape, comprising a center core portion of high refractiveindex, a step core portion of lower refractive index than said centercore portion, provided about outer circumference thereof; and clad oflower refractive index than said step core portion, provided about outercircumference of the step core portion; wherein the dispersion-shiftedoptical fiber has, in a used wavelength band selected from 1490 to 1625nm, Aeff of 45 to 90 μm², dispersion slope of from 0.05 to 0.14ps/km/nm², bending loss of 100 dB/m or less, and chromatic dispersionvalue of either from −0.5 to −8.0 ps/km/nm or from +0.05 to +10.0ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; wherein smaller diameter solutionis adopted for said core diameter, said dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 65to 95 μm², dispersion slope of from 0.08 to 0.14 ps/km/nm², bending lossof 100 dB/m or less, and absolute values of chromatic dispersion valueof from 0.5 to 8.0 ps/km/nm, and has a cutoff wavelength such thatsubstantially single-mode propagation is realized; and wherein, whenradius of said center core portion is represented as r1, radius of saidstep core portion as r2, relative refractive-index difference of saidcenter core portion when refractive index of said clad is taken asreference as Δ1, relative refractive-index difference of said step coreportion as Δ2, r2/r1 as x, and Δ2/Δ1 as y, 5≦x≦10, 0.08≦y≦0.22, and0.6%<Δ1<1.2%.
 8. A dispersion-shifted optical fiber having a refractiveindex distribution shape, comprising a center core portion of highrefractive index, a step core portion of lower refractive index thansaid center core portion, provided about outer circumference thereof,and clad of lower refractive index than said step core portion, providedabout outer circumference of the step core portion; wherein thedispersion-shifted optical fiber has, in a used wavelength band selectedfrom 1490 to 1625 nm, Aeff of 45 to 90 μm², dispersion slope of from0.05 to 0.14 ps/km/nm², bending loss of 100 dB/m or less, and chromaticdispersion value of either from −0.5 to −8.0 ps/km/nm or from +0.05 to+10.0 ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; wherein smaller diameter solutionis adopted for said core diameter, said dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 65to 95 μm², dispersion slope of from 0.08 to 0.14 ps/km/nm², bending lossof 100 dB/m or less, and absolute values of chromatic dispersion valueof from 0.5 to 8.0 ps/km/nm, and has a cutoff wavelength such thatsubstantially single-mode propagation is realized; and having a zerodispersion wavelength on the side of longer wavelengths than thewavelength band used.
 9. The dispersion-shifted optical fiber accordingto claim 8, wherein, when radius of said center core portion isrepresented as r1, radius of said step core portion as r2, relativerefractive-index difference of said center core portion when refractiveindex of said clad is taken as reference as Δ1, relativerefractive-index difference of said step core portion as Δ2, r2/r1 as x,and Δ2/Δ1 as y, 6≦x≦7, 0.1≦y≦0.18, y≧(−0.02x+0.24), 0.6%≦Δ1≦1.2%, Aeffis from 65 to 75 μm², and dispersion slope is 0.125 ps/km/nm² or less.10. The dispersion-shifted optical fiber according to claim 8, wherein,when radius of said center core portion is represented as r1, radius ofsaid step core portion as r2, relative refractive-index difference ofsaid center core portion when refractive index of said clad is taken asreference as Δ1, relative refractive-index difference of said step coreportion as Δ2, r2/r1 as x, and Δ2/Δ1 as y, 7≦x≦8, 0.1≦y≦0.16,y≧(−0.016x+0.21), 0.6% ≦Δ1≦1.2%, Aeff is from 70 to 80 μm², anddispersion slope is 0.130 ps/km/nm² or less.
 11. The dispersion-shiftedoptical fiber according to claim 8, wherein, when radius of said centercore portion is represented as r1, radius of said step core portion asr2, relative refractive-index difference of said center core portionwhen refractive index of said clad is taken as reference as Δ1, relativerefractive-index difference of said step core portion as Δ2, r2/r1 as x,and Δ2/Δ1 as y, 7≦x≦8.5, 0.1≦y≦0.16, (−0.02x+0.26)≦y≦(−0.02x+0.32),0.6%≦Δ1≦1.2%, Aeff is from 75 to 85 μm², and dispersion slope is 0.135ps/km/nm² or less.
 12. A dispersion-shifted optical fiber having arefractive index distribution shape, comprising a center core portion ofhigh refractive index, a step core portion of lower refractive indexthan said center core portion, provided about outer circumferencethereof; and clad of lower refractive index than said step core portion,provided about outer circumference of the step core portion; wherein thedispersion-shifted optical fiber has, in a used wavelength band selectedfrom 1490 to 1625 rim, Aeff of 45 to 90 μm², dispersion slope of from0.05 to 0.14 ps/km/nm² bending loss of 100 dB/m or less, and chromaticdispersion value of either from −0.5 to −8.0 ps/km/nm or from +0.05 to+10.0 ps/km/nm, and has a cutoff wavelength such that substantiallysingle-mode propagation is realized; wherein smaller diameter solutionis adopted for said core diameter, said dispersion-shifted optical fiberhas, in a used wavelength band selected from 1490 to 1625 nm, Aeff of 65to 95 μm², dispersion slope of from 0.08 to 0.14 ps/km/nm², bending lossof 100 dB/m or less, and absolute values of chromatic dispersion valueof from 0.5 to 8.0 ps/km/nm, and has a cutoff wavelength such thatsubstantially single-mode propagation is realized; and having a zerodispersion on the side of shorter wavelength than the wavelength bandused.
 13. The dispersion-shifted optical fiber according to claim 12,wherein, when radius of said center core portion is represented as r1,radius of said step core portion as r2, relative refractive-indexdifference of said center core portion when refractive index of saidclad is taken as reference as A1, relative refractive-index differenceof said step core portion as Δ2, r2/r1 as x, and Δ2/Δ1 as y, 5≦x≦8,0.12≦y≦0.22, (−0.02x+0.24)≦y≦(−0.02x+0.34), 0.6%≦Δ1≦1.2%, Aeff is from65 to 75 μm², and dispersion slope is 0.110 ps/km/nm² or less.
 14. Thedispersion-shifted optical fiber according to claim 12, wherein, whenradius of said center core portion is represented as r1, radius of saidstep core portion as r2, relative refractive-index difference of saidcenter core portion when refractive index of said clad is taken asreference as Δ1, relative refractive-index difference of said step coreportion as Δ2, r2/r1 as x, and Δ2/Δ1 as y, 5.5−x≦8, 0.12≦y≦0.20,(−0.02x+0.25)≦y≦(−0.02x+0.33), 0.6%≦1≦1.2%, Aeff is from 70 to 80 μm²,and dispersion slope is 0.1 15 ps/km/nm² or less.
 15. Thedispersion-shifted optical fiber according to claim 12, wherein, whenradius of said center core portion is represented as r1, radius of saidstep core portion as r2, relative refractive-index difference of saidcenter core portion when refractive index of said clad is taken asreference as Δ1, relative refractive-index difference of said step coreportion as Δ2, r2/r1 as x, and Δ2/Δ1 as y, 6≦x≦8, 0.12≦y≦0.20,(−0.02x+0.26)≦y≦(−0.02x+0.35), 0.6%≦Δ1 Δ1.2%, Aeff is from 75 to 85 μm²,and dispersion slope is 0.125 ps/km/nm² or less.